A major application of the method and apparatus of the present invention is in the analysis of the composition of semi-crystalline polymers such as the analysis of short chain branching distribution (SCBD) in linear low density polyethylenes. This analysis is presently typically carried out by techniques such as Temperature Rising Elution Fractionization (TREF) or Analytical Temperature Rising Elution Fractionization (ATREF). See L. Wild and T.R. Ryle, Polymer Preprints; Am.Chem.Soc.Polym.Chem.Div. 18,182 (1977). It is also known to combine ATREF with High Temperature Continuous Viscometry (HTCV) for analysis of polymers as is disclosed in U.S. Pat. No. 4,798,081 Jan. 17, 1989 to Hazlett et al.
Although TREF and ATREF techniques can be used for the analysis of polymers, these techniques are labor intensive and time consuming. In addition, the TREF and ATREF techniques require relatively complex apparatus. In the TREF and ATREF techniques a first sequential crystallization step is carried out to fractionate the sample in terms of branching content and then a second step is carried out in which the fractions are separated and analyzed to obtain the short chain branching distribution of the polymer. Typically, this analysis is carried out by crystallizing a solution as the temperature goes down which segregates the various fractions according to crystallinity. Once the crystallization has been completed, the short chain branching distribution information is contained in the segregated crystals, but an elution step must be carried out to retrieve and visualize the information. In the elution step, the temperature cycle is reversed and samples are eluted and analyzed for concentration as the temperature increases. The crystallization step often takes more than one day to carry out. The elution step adds further time to the method.
It will be appreciated that it is often highly desirable to complete an analysis as quickly as possible. It will also be appreciated that it is often desirable to process a large number of samples in a short time period. Of course, it is generally desirable to have a technique which is straightforward, which is not labor intensive, and which can be carried out using relatively inexpensive apparatus. For some uses it is desirable to have a technique which can be readily implemented in a quality control lab.
While the technique of the present invention is especially useful in composition distribution analysis in copolymers, broadly speaking, the technique provides a novel approach for analysis of the crystallization or precipitation of soluble compounds. Thus, the technique of this invention can be used to study crystallization kinetics, for solubility and extractable analysis of any solid chemical in a solvent, or for solvent elution analysis (molecular weight distribution) of polymers.
In accordance with the present invention, analysis is carried out at the same time that crystallization takes place by analyzing the remaining solution concentration as the solute precipitates out of solution. Hence, the analysis can be carried out relatively quickly. The present technique facilitates the processing of a relatively large number of samples using relatively inexpensive apparatuses and could be readily implemented in a quality control lab. Furthermore, this technique can be used for isothermal crystallization studies of polymer solutions or solutions of other chemical compounds. The technique provides a new approach to study solid/solvent interactions. Solubility and extractables can be easily measured automatically at various temperatures and, by using a solvent elution gradient, molecular weight distribution of polymers could also be measured.
Further understanding of the present invention will be had from the following description taken in conjunction with the accompanying claims and drawings. All percentages and parts herein are by weight unless specifically indicated otherwise.